A Variable Frequency Bidirectional CLLC Resonant Converter with Variable Capacitor

Isolated bidirectional CLLC resonant converter generally operates at a resonant frequency to ensure desired active power transmission ratio, high efficiency and stable voltage gain. However, its actual resonant frequency usually deviated from the operating frequency is not constant due to parameter deviations of the resonant components and variations of the operation power and temperature, which may change its voltage conversion gain and transmission efficiency. This paper proposes a novel variable frequency isolated bidirectional resonant converter with variable capacitor clamp auxiliary circuits. DC voltage bias on ceramic capacitors are applied and the resonant capacitance is changed by adjusting duty cycles of the auxiliary switches, which are almost lossless. DC voltage bias characteristics of ceramic capacitors and output characteristics of the converter are researched. Experimental results obtained from a prototype verify the theoretical analysis of the proposed novel bidirectional CLLC resonant converter.